1. Field of the Invention
The present invention generally relates to a high-frequency filtering technique in filtering circuit and a structure thereof.
2. Description of Related Art
In a communication system, signals in all other bands except the operation band are considered interferences, and these interferences may affect the communication quality of the system. Accordingly, a filter is usually disposed in a communication system for passing signals in the operation band and filtering out those signals in other bands. After a signal in the operation band is passed through a filter, the power loss of the signal has to be kept at a low level. In other words, a signal in the operation band passed through the filter has to be close to the original signal. The signals out of the operation band have to be effectively suppressed by the filter in order to ensure a good communication quality of the system.
In a planar circuit, microstrips or striplines are usually used for implementing a filter. FIG. 1 is a circuit diagram of a conventional quarter wavelength inter-digital coupled-line filter implemented with microstrips. Referring to FIG. 1, the filter 100 receives a signal through an input terminal Tin and then sequentially transmits the signal to an output terminal Tout through N coupled lines 130_1˜130_N. The coupled lines 130_1˜130_N are all microstrips of quarter wavelength, wherein one terminals of the coupled lines 130_1˜130_N are grounded, and the other terminals thereof are open. Since the coupled lines 130_1˜130_N are equivalent to resonators composed of capacitors and inductors, the filter is filtered by a plurality of capacitors and inductors when the signal is sequentially transmitted to the output terminal Tout through the coupled lines 130_1˜130_N. FIG. 2 is a circuit diagram of another conventional quarter wavelength inter-digital coupled-line filter implemented with microstrips. Referring to FIG. 2, the circuit structure of the filter 200 is similar to that of the filter 100 illustrated in FIG. 1. The difference is that in the filter 200, the input terminal Tin is connected to an input transmission line 210, and the input transmission line 210 is directly plugged into the first microstrip 230_1. Besides, the output transmission line 220 is directly plugged into the last microstrip 230_N.
In foregoing two filters 100 and 200, three methods are adopted for increasing the coupling from input terminal to output terminal through the coupled lines, including reducing the line widths of the coupled lines, increasing the thickness of the substrate; and reducing the gap width between the coupled lines. However, reduction in the line widths of the coupled lines may reduce the quality factor of the resonators and accordingly increase the transmission loss of the resonators. The effect brought by increasing the thickness of the substrate is very limited, and under the trend of slimming circuit boards, thick substrates have become outdated. The method of reducing the gap width between the coupled lines is the most effective one; however, the smaller the gap width between the coupled lines is, the greater negative affections resulted from the variation of a circuit board fabrication process for the small gap width. FIG. 3 illustrates the affection of the gap width between parallel coupled microstrips to signal coupling with fixed substrate thickness, substrate dielectric coefficient, and line width. As shown in FIG. 3, the smaller the gap width is, the more the signal coupling changes. Accordingly, a slight process variation can deviate the response of a filter away from the original design and accordingly reduce the yield of filters in mass production.
A band-pass filter with quarter wavelength transmission lines as illustrated in FIG. 4 has been disclosed in European patent. NO. WO 2006/095984 A1. Referring to FIG. 4, the band-pass filter 400 includes an input terminal 410, an output terminal 420, resonators 431˜433, and transmission lines 441˜442. The couplings between foregoing components are as illustrated in FIG. 4. In the band-pass filter 400, an input signal is sequentially filtered by the input terminal 410, the resonator 431, the transmission line 441, the resonator 432, the transmission line 442, the resonator 433, and the output terminal 420. Even though this band-pass filter can filter signals effectively, it cannot suppress sideband interferences effectively regarding the frequency response thereof.